Foam-backed pressure-sensitive adhesive tape is commonly used to adhere an article to a substrate. The foam backings of such tapes often are pigmented with carbon black to camouflage their presence.
The above-cited Levens patent discloses a pressure-sensitive adhesive tape which has a foamlike appearance and character, even though it is not a foam, and is useful for purposes previously requiring a foam-backed pressure-sensitive adhesive tape. A foamlike tape of the Levens patent now on the market is made by ultraviolet polymerization of a layer of an adhesive-forming mixture containing colorless glass microbubbles which afford a white color that makes the tape undesirably visible in uses such as sealing a skylight or attaching body-side moldings to automotive vehicles or simulated mullion bars to a glass window. Because of its superior performance characteristics, the foamlike tape of the Levens patent is often preferred to foam-backed tapes for such purposes and would be more acceptable if its adhesive layer were sufficiently dark to camouflage the tape. If carbon black or other pigment were added to the photopolymerizable adhesive-forming mixture in amounts sufficient to produce a desirably dark appearance, this would block the ultraviolet radiation from polymerizing the mixture to a pressure-sensitive adhesive state. Up to about 0.1 or 0.15 percent by weight of carbon black can be employed without undue interference with the polymerization of a 50-micrometer layer, but this results in a pastel gray color that would be undesirably noticeable for most uses such as those mentioned above.
Glass microbubbles also are used as fillers for other polymeric articles to which they afford lighter weight in addition to advantages provided by other inert fillers, e.g., higher distortion temperatures. Lighter weight is of special importance to automotive body parts which also should be pigmented internally in order to mask from view the mechanisms which they cover.
Glass microbubbles can be made according to U.S. Pat. Nos. 3,365,315 (Beck et al.) or 4,391,646 (Howell). Individual microbubbles of the working examples of those patents are colorless.